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722

The Pelton Wheel.

leaves the wheel with little absolute velocity, and the efficiency
is about '66, a great improvement over that of the old radial-float
wheel, which was only '3. As the water never fills the vanes,
there is no pressure, but pure impulse only, and the efficiency is
therefore constant under varying sluices. Horse-power may be
reckoned from head or velocity (see pp. 719 and 720). The
circumferential velocity is about "55 of that due to head, and the
jet thickness is about 8 or 10 ins. The wheel is suitable for falls
up to 6 feet, and the diameter may be four times the fall.

The Pelton Wheel, Fig. 708, is an American machine, in which
a small jet issues from a nozzle A, with great head, and impinges
on a series of cups B B, of the form of a split semicircle in end
elevation c, and simply cup-form in side elevation D. In this
way the jet, about f" diameter, is split, and returned without
serious shock. In one example 320 H. P. was given off from a
fall of 523 ft., the nozzles being one inch diameter. The efficiency
is commonly '8, but may reach *p.                                  " -